Method of fabricating a carbon-fiber reinforced composite article

ABSTRACT

Light weight composite refractory articles comprising a plurality of metal or alloy coated carbon fibers bonded together by an aluminum base matrix are provided. The coating metal or alloy can be any material selected from the group consisting of silver, silver-aluminum base alloys and mixtures thereof. The preferred process for producing such composites comprises contacting silver coated carbon fibers with an aluminum base material followed by hot pressing the so-contacted fibers at the solidus temperature of the aluminum base material to infiltrate aluminum around the silver-coated fibers and then cooling the resultant article to a completely solidified state. Such composite articles are characterized by a high tensile strength coupled with a high modulus of elasticity and are especially suited as materials of construction for aerospace and hydrospace vehicles and systems.

United States Patent [72] Inventor Raymond Vincent Sara North Olmsted,Ohio [21 Appl. No. 833,009 [22] Filed June 13, 1969 Division ot S er.No. 727,898, May 9, 1968. [45] Patented Mar. 23, 1971 [73] AssigneeUnion Carbide Corporation New York, N.Y.

[54] METHOD OF FABRICATING A CARBON-FIBER REINFORCED COMPOSITE ARTICLE3,491,055 l/1970 Talley Att0rneysPaul A. Rose, Robert C. Cummings,Frederick J.

Mc Carthy, Jr. and John S. Piscitell ABSTRACT: Light weight compositerefractory articles com- 5 Claims, 2 Drawing Figs.

prising a plurality of metal or alloy coated carbon fibers [52] US. Cl29/419, bonded together by an aluminum base matrix are provided,

/8 164/91 The coating metal or alloy can be any material selected from[51] Int. Cl B23p 17/0 the group consisting of silver, silver-aluminumbase alloys and [50] Field of Search 164/80, 91, i t th f, Th eferredprocess for producing such 29/182-2, 1325; 29/419, 527-3, 5275composites comprises contacting silver coated carbon fibers with analuminum base material followed by hot pressing the [56] ReferencesCited so-contacted fibers at the solidus temperature of the alu- UNITEDSTATES PATENTS minum base material to infiltrate aluminum around thesilver- 9l0,674 1/1909 Hancock 164/86 coated fibers and then cooling theresultant article to a 2,937,436 5/1960 Butler et a1. 29/419 completelysolidified state. Such composite articles are 3,038,248 6/1962 Kremer29/419 characterized by a high tensile strength coupled with a high3,187,422 6/1965 Morgan 29/419 modulus of elasticity and are especiallysuited as materials of 3,256,596 6/ 1966 Fiedler 29/419 construction foraerospace and hydrospace vehicles and 3,476,529 1 1/1969 Dubin et a1. 117/71X systems.

A lummum PATENIED m2 3 I9?! Aluminum Carbon Silver-Aluminum INVENTOR.RAYMOND V. SARA A T TORNE V METHOD OF FABRICATING A CARBON-FIBERREINFORCED COMPOSITE ARTICLE This application is a division of copendingapplication Ser. No. 727,898, filed May 9, 1968.

BACKGROUND OF THE INVENTION 1 Field of the Invention The presentinvention relates to high strength, high modulus composite refractoryarticles composed of a plurality of carbon fibers which are essentiallycoated with a thin layer of silver and/or silver base alloys and bondedtogether by an aluminum base matrix and to the process of producing thesame.

2. Description of the Prior Art In the space and missile industries,there has developed a need for refractory materials of constructionwhich exhibit exceptional physical properties, for example, low densitycoupled with high strength and stiffness. Attempts to produce suchrefractory materials have centered around the fabrication of compositearticles.

One of the most promising materials available today for use in compositeform is carbon textiles since they possess excellent refractoryproperties and are commercially available in all known textile forms.Today, it is well known to form composites of carbon textiles andresins.

Recently, efforts have been directed to forming composites of carbontextiles and metals. The prime object of forming such composites is toincrease the strength of the metal matrix by the inclusion therein ofhigh strength carbon fibers.

Aluminum has been suggested as the matrix media for carbon fiber-metalcomposites which are intended for use in aerospace applications, in themain, due to its low density. However, attempts heretofore toincorporate carbon fibers in an aluminum matrix have met with little orno practical success due to the fact that carbon, especially its graphicform, is not readily wetted by molten aluminum.

The present invention overcomes the foregoing problem by employing anintermediate coating or wetting agent which when applied to the carbonfibers enables them to be readily bonded together by an aluminum basematrix material.

SUMMARY Broadly stated, the carbon fiber-aluminum composite article ofthe invention comprises a plurality of carbon fibers each of which iscoated with either a thin layer of silver, a silver-aluminum base alloyor mixtures thereof and bonded together, preferably in a side-by-side orparallel manner, with an aluminum binder or matrix material. Generally,this composite article may be provided by a process which comprisescoating carbon fibers with a thin but essentially continuous film ofsilver, contacting the so-coated fibers with a solidified aluminum basematerial, hot pressing the so-produced assembly at the solidustemperature of the aluminum base material to infiltrate it around thecoated fibers and cooling the resultant aluminum bonded carbon fibers toproduce a composite article. This article can then be formed into anydesired shape by known techniques which will readily suggest themselvesto those skilled in the art.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatical illustration of aportion of a carbon fiber-aluminum composite article produced accordingto the preferred teachings of the invention; and

FIG. 2 is a greatly magnified diagrammatical illustration of a singlefiber found in the composite of FIG. 1.

Referring now to the drawings in detail, FIG. 1 shows a portion of arectangular composite article 10 consisting of aligned graphite fibers12 (the silver coating and aluminum-silver zone thereon are not shown)bonded together by an aluminum matrix 18. The graphite fibers 12 aredisposed in the aluminum matrix in a parallel manner with their lengthdimension being perpendicular to the surface of the drawing. FIG. 2shows a single graphite fiber 12 taken from the composite l of FIG. 1having on its surface a coating of silver 14 and an aluminum-silverinterface zone 16 between the silver coating and the aluminum matrix 18.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Carbon textilesin any form can be employed in the practice of the instant invention.However, it is preferred to employ carbon fibers in yarn ormultifilament form. Carbon textiles are available commercially and aregenerally produced by the techniques described in U.S. Pat. Nos.3,107,152 and 3,1 16,975, among others.

Silver can be deposited on the carbon fibers by a variety of methods.The techniques available for accomplishing this in cludeelectrodeposition from a conductive bath, reduction of salts to themetal or sputtering. The exact deposition technique to be employed isdictated by a number of factors. Sputtering can be used on relativelycomplex shapes and results in a tenacious bond between the thin silvercoating and the carbon fiber substrate. Such a bond is a highlydesirable feature in carbon fiber-metal matrix composites. Dipping ofthe carbon fibers into a silver containing solution and chemicallyreducing the salt to the metal can also be employed, however, with thistechnique it is difficult to accurately control the thickness of thesilver coating. Electrodeposition of silver from a conductive bath is anideal way of coating carbon fibers with a thin film of silver and is thepreferred method since it results in a uniform, tenaciously bondedmetallic coating.

The following example illustrates in detail the preferred practice ofthe instant invention.

A single ply graphite yarn composed of 720 monofilaments, eachcharacterized by an average modulus of elasticity of 3 3.5 X 10 lb./in.and an average tensile strength of 20 X 10 lb./in. was cut into aplurality of 4inch lengths. These lengths of yarn were heated at 560 C.for 30 seconds in a tube furnace which was open to the atmosphere. Theslightly oxidized fibers were immediately plated electrolytically withsilver by placing them in a strike solution for about 5 to 10 secondsand then into a conventional plating solution for about 5 to 10 seconds.The plating current employed was about 400 milliamps for the strikesolution and between about 600 to 800 milliamps for the conventionalplating solution. The make up of the strike and conventional platingsolution was as follows:

Strike Solution Silver Cyanide (Ag CN) 6.5 g./ 1. Potassium Cyanide(KCN) 68.0 g./ 1.

Regular Solution Silver Cyanide (Ag CN) 41 g./1. Potassium Cyanide (KCN)4O g./1. Potassium Hydroxide (K011) 11 g./1. Potassium Carbonate (K CO62 g./l.

The so-coated graphite fibers were cut into one inch lengthsand thenpositioned in a parallel manner between alternate layers of aluminumalloy foils measuring'0.003 inch X 0.125

inch X 1.00 inch each. The aluminum alloy employed con sisted of 4.5weight percent copper, 0.6 weight manganese, 1.5 weight percentmagnesium with the remainder being essentially aluminum. Two singleplies of silver-coated yarn were used for each layer. The complete arrayconsisted of 22 layers of foil and 42, one-inch lengths of silver-coatedsingle ply yarn. This assembly was placed in a graphite mold and hotpressed at 2,250 lb./in. in vacuum at approximately 550 C. for one hour.The composite was subsequently cooled in the mold to room temperatureand then formed into a 1/16" XVs" X 1" specimen for physical propertymeasurements. This specimen evidenced an average tensile strength of44,10 lb./in. and a Youngs modulus of 13.0 X 10 lb./in.

The table below shows the improvement obtained by incorporating silvercoated graphite fibers in an aluminum base matrix material by thetechnique of the instant invention.

TABLE PROPERTIES OF GRAPHITE-FIBER, ALUMINUM COMPOSITE Compositesproduced by the technique of the instant invention are extremely usefulas materials of construction for subsonic and supersonic aircraft, spacesystem components and various propulsion devices.

It will be appreciated by those skilled in the art that while thepreferred technique of the invention results in a composite articlewhich comprises silver-coated carbon fibers bonded together by analuminum base matrix, the silver coating on the carbon fibers may beconverted to a silver-aluminum base alloy by annealing the compositeduring its fabrication or after it has been formed. The only requirementbeing that the silveraluminum base alloy not be formed at a temperaturein excess of the melting point of the applied silver coating.Accordingly, it is clear that the instant invention results in a uniquetype of composite comprising carbon fiber which are coated with.

either silver, a silver-aluminum base alloy or mixture thereof andbonded together by an aluminum base matrix.

While the foregoing example concerns a composite having the fiberspositioned in a side-by-side relationship, it is readily apparent tothose skilled in the art that the carbon fibers may be randomlyorientated in the aluminum matrix if more isotropic physical propertiesare desired without losing the benefits of the instant invention. Inaddition, it is obvious that the thickness of the silver can be variedas desired. All that is required is that it be thick enough to preventthe aluminum matrix metal from coming into contact with the reinforcingcarbon fibers to such an extent that there is essentially no bondingbetween the fibers and the matrix metal. Likewise, it

will be appreciated by those versed in the art that although graphitefibers and fabrics are preferred in the practice of the instantinvention, nongraphitic carbon fibers and fabrics may also be employed.

It should be noted here that while the invention can be beneficiallypracticed by using pure aluminum as the matrix metal, it is preferred toemploy an aluminum base alloy which has an incongruent meltingtemperature. Such alloys are well known in the art and will not bediscussed herein in detail.

In addition it should be noted that while it may be possible toinfiltrate the molten aluminum metal around the silver clad carbonfibers without removing the silver therefrom, the preferred practice ofthe invention involves infiltrating the matrix metal around thefilaments by heating the metal to a temperature at which it begins tomelt but below that at which 7 it becomes completely liquid, i.e., atits solidus temperature,

and then forcing the so-softened matrix metal around the individualfilaments by the means of an applied pressure. The amount of pressure tobe applied is not particularly critical and need only be sufficient tocompact the fibers and matrix material into an essentially nonporousarticle. 4

The term carbon as used herein and in the appended claims vis meant toinclude both the nongraphitic and graphitic forms of carbon.

The foregoing example is presented for illustrative purposes only and isnot intended to unduly limit the reasonable scope of the instantinvention. The limitations of applicants invention are defined by thefollowing claims.

I claim:

1. A method of producing a metal coated carbon fiber aluminum-bondedcomposite article comprising:

a. contacting a plurality of silver coated carbon fibers with asolidified aluminum base material;

b. heating the so-formed mass to the solidus temperature of magnesiumwith the remainder being essentially aluminum.

3. A method of producing an aluminum-bonded silvercoated carbon fibercomposite article comprising:

a. forming an assembly comprising alternate layers of silver coatedcarbon fibers and aluminum base foil, said fibers being positionedthereon in a parallel manner;

b. placing the so-formed assembly in mold and heating it in an inertatmosphere to a temperature approximately equal to the solidustemperature of said aluminum base foil while exerting a sufficientpressure on said assembly to cause said aluminum base foil material tobe infiltrated around said silver-coated carbon fibers; and

c. cooling the resultant assembly to produce an aluminumbondedsilver-coated carbon fiber composite.

4. The method of claim 3 wherein:

a. said aluminum base foil material consists essentially of 4.5 weightpercent copper, 0.6 weight percent manganese, 1.5 weight percentmagnesium with the remainder being essentially aluminum; and

b. said heating in step (b) is accomplished in a vacuum at a temperatureof about 550 C.

5. The method of claim 1 wherein said aluminum-bonded silver-coatedcarbon fiber composite article is heated to a temperature sufficient tocause at least some said silver to alloy with at least some of saidaluminum to form at least a partial coating of a silver-aluminum basealloy on said fibers.

2. The method of claim 1 wherein said aluminum base material is an alloyconsisting essentially of 4.5 weight percent copper, 0.6 weight percentmanganese, 1.5 weight percent magnesium with the remainder beingessentially aluminum.
 3. A method of producing an aluminum-bondedsilver-coated carbon fiber composite article comprising: a. forming anassembly comprising alternate layers of silver coated carbon fibers andaluminum base foil, said fibers being positioned thereon in a parallelmanner; b. placing the so-formed assembly in mold and heating it in aninert atmosphere to a temperature approximately equal to the solidustemperature of said aluminum base foil while exerting a sufficientpressure on said assembly to cause said aluminum base foil material tobe infiltrated around said silver-coated carbon fibers; and c. coolingthe resultant assembly to produce an aluminum-bonded silver-coatedcarbon fiber composite.
 4. The method of claim 3 wherein: a. saidaluminum base foil material consists essentially of 4.5 weight percentcopper, 0.6 weight percent manganese, 1.5 weight percent magnesium withthe remainder being essentially aluminum; and b. said heating in step(b) is accomplished in a vacuum at a temperature of about 550* C.
 5. Themethod of claim 1 wherein said aluminum-bonded silver-coated carbonfiber composite article is heated to a temperature sufficient to causeat least some said silver to alloy with at least some of said aluminumto form at least a partial coating of a silver-aluminum base alloy onsaid fibers.